It has been found that if two transparent plates are each rubbed in a single direction and the two plates are mounted opposite each other with the directions of rubbing at right-angles to each other and with the rubbed surfaces facing each other, and that if the space between the plates is filled with a liquid composition having a sufficiently high concentration of a nematic liquid material having a sufficiently high positive dielectric anisotropy, then such a cell has the property of rotating the plane of polarized light falling thereon by 90.degree.. Furthermore, if a sufficiently high electric field is imposed across such a cell, then the optical activity of the cell is decreased to zero for the period during which the voltage is imposed. When the voltage is removed or cut below the threshold voltage then the optical activity of the cell is restored.
Placing a polarizer plate at one face of the cell and an analyzer plate at the other converts the system into an electrically driven optical shutter. If the axes of the polarizer and analyzer plates are parallel, then in the absence of a suitable electrical field, no light will be transmitted through the device. Conversely, if the axes of the polarizer and analyzer plates are at right-angles to each other, then light will be transmitted through the device only in the absence of an electric field.
Simple as the device appears to be in principle, a number of difficulties arise in attempting to make a practical system. Materials having high positive dielectric anisotropy tend to be colored, so that if they are used in high concentration the system will transmit little light even when the "shutter" is open. Further, it is necessary that the nematic liquid crystal composition have a mesomorphic phase range which includes room temperature, else it will be difficult to utilize the device. Also, the nematic liquid crystals must be stable under the conditions of use and the composition should have a low threshold voltage. It is particularly to be desired that the threshold voltage be no greater than 3.0 volts thus making it possible for a two-cell battery to operate the system. Moreover, the electrical resistivity of the system must be high in order to avoid exhausting the power source too rapidly.